X-ray generating systems typically include an electron generating cathode and an anode assembly in a sealed housing. The cathode provides an electron stream or current that is directed towards the anode assembly. This focused electron beam is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode. Because of the high power density generated at the location where the electron beam strikes the anode, it is desirable to rotate the anode assembly. Many X-ray tubes therefore include a rotating anode structure for distributing the heat generated at a focal spot.
The requirements for X-ray tubes used in computed tomography have steadily grown with the manifold possibilities of computed tomography. Modern computed tomography systems require X-ray tubes that allow the X-ray current thereof to be modulated with high speed, for example, to be able to achieve an optimized dose modulation or operation at two different energies with an equilibrium photon flow (flux).
One of the limitations associated with the high power imaging X-ray tubes described in the prior art is unavailability of fast gridding or current modulation. In order to change the electron beam current, the method suggested in the prior art is to adjust the temperature of a filament in the cathode. However, this is a slow process and the time scale of changing the temperature is in millisecond range. This fails to keep pace with view-to-view frame change, where the requirement for current modulation is in a span of microseconds.
Another method described in the prior art involves placing an aperture plate opposite to cathode in order to change the emission. The voltage changes on the aperture plate influences the emission from the cathode. However, one issue related to this method is the focal spot change when the emission current is modulated. By using a mesh grid, the focal spot size change can be reduced. Meshes to grid electron emission are e.g. used in microwave amplifiers. The mesh grid controls the path of the electron beam and focuses the beam. The time response is greatly improved by the addition of the mesh grid to control the emitted electrons from the cathode by means of varying the potential applied at the mesh grid.
The mesh grid typically comprises a two-dimensional high transparency grid with uniform mesh spacing in both dimensions. Providing such mesh grid minimizes the interception of beam electrons by increasing the mesh transparency. However, beam electrons may interact with the mesh grid, across the total beam cross section. This has the undesirable effect of beam loss, beam degradation and mesh degradation.
Moreover, the mesh grids employed are typically positively biased with respect to the cathode. Consequently, electrons emitted from the cathode may reach the mesh grid and degrade the mesh grid by overheating due to electron bombardment.
Therefore, it would be desirable to have an apparatus and method for minimizing the voltage necessary for extraction of the electron beam from the cathode, while still allowing fast modulation of the beam current for sufficient focusing of the electron beam so as to form a usable focal spot on a target. In particular, it would be desirable to have a mesh grid that allows for efficient low voltage extraction and beam focusing.